The thermal efficiency of an Otto cycle internal combustion engine depends on the compression ratio. An increase in the compression ratio increases the thermal efficiency of the engine and frequently entails an increase in the tendency of the engine to knock. Compression ratios of the spark ignition engine are now limited by knock.
The knock in internal combustion engines is associated with the sounds that are created by the engines. Knock associated phenomena can sometimes destroy an engine within minutes. Destructive knock may be associated with detonation during the combustion of the fuel in the combustion chamber.
Flames in the combustion chamber may propagate through the combustible mixtures either as deflagrations or as detonations or they may originate at spontaneous ignition sites. Deflagrations are subsonic and associated with small spatial pressure variations. Detonations are supersonic. They are associated with large pressure discontinuities and impact pressures. The pressure discontinuities and impact pressures can cause the damage associated with knock.
Conventional combustion chamber design is such as to promote turbulence. The cross sectional area of the combustion chamber decreases in the region of the end gas to produce a quench or squish zone. Without turbulence, the highly agitated motion of the fuel air mixture, slow combustion would result in inefficient operation of the engine. This shape of the combustion chamber does not inhibit detonation.
Detonation in an internal combustion engine chamber produces sound and pressure stresses. Various devices have been proposed to eliminate detonation by attenuating the high amplitude of these pressure stresses. Bodine, in U.S. Pat. No. 2,760,472, utilizes a sound wave absorber pad between the block and the head to attenuate the high amplitude detonation into sound waves. The piston has a truncated inverted cone shape. Kydel et al. discloses, in U.S. Pat. No. 2,826,185, an internal combustion engine having a piston equipped with a projection. The projection is mounted on top of the mid-section of the piston and has downwardly and outwardly sloping flat surfaces. The head is provided with a firing chamber that decreases in size toward the center of the piston. Polza, in U.S. Pat. No. 2,969,786, shows an internal combustion engine having a piston with an angularly related face providing a firing chamber adjacent to a spark plug. Burnham, in U.S. Pat. No. 4,046,116, shows a piston for an internal combustion engine carrying a plate to increase the compression ratio of the engine. The plate has an upwardly sloping side wall facing the valves to provide clearance for the valves. Takeshi, in U.S. Pat. No. 4,162,661, shows a piston for an internal combustion engine having two separate raised portions located at two peripheral portions on the top of the piston. The ends of the raised portions have concave surfaces to provide for mixing of an air/fuel mixture to enhance combustion and increase engine output power. Thery, in U.S. Pat. No. 4,235,203, shows a two-zone combustion chamber formed by a piston having an upwardly directed projecting part that divides the combustion chamber into two portions. The projecting part has a channel providing communication between the parts of the combustion chamber. These piston structures and combustion chamber shapes have some effect on detonation, but do not control detonation to allow high compression ratios without damage to the piston and head.